Developer devices for visualizing (developing) an electrostatic latent image formed on the surface of a photoreceptor using a developer material composed of carrier granules and toner particles are used, for example, in many dry-type copying machines. In such developer devices, the quantity of the toner particles is reduced while being used in the developing process; whereas, the quantity of carrier granules in the developer material remains the same. Therefore, the quality of the carrier granules being stirred with toner particles in the developer material deteriorates because a resin coating layer on the surface thereof is peeled, or toner particles adhere onto the surface thereof. As a result, the charging ability of the developer material gradually deteriorates.
The device which prevents the deterioration of the charging ability by supplying additional carrier granules separately from the refill for the used toner particles has been disclosed (see, for example, Japanese Laid-Open Patent Application No. 21591/1990 (Tokukouhei 2-21591)). In such a device, when adding additional carrier granules, excessive developer material in the developer vessel is discharged through a discharge opening formed on the wall of the developer vessel to be collected in a waste container. By repeating the above refill and discharge of the developer material, the developer material in the developer vessel whose charging ability has deteriorated can gradually replace. Thus, the required charging ability of the developer material can be restored, and the deterioration in the copied image quality can be prevented.
However, when the carrier granules are additionally supplied to the developer material little by little, the quantity of developer material in the developer vessel is likely to change, and thus the ratio of toner particles to the developer material cannot be accurately controlled. Therefore, the above device presents the problem that it is difficult to ensure a desirable image quality.
In the above device, additional toner particles are supplied by detecting the ratio of toner particles in the developer vessel, for example, using the toner concentration sensor composed of a magnetic permeability sensor. In the case where carrier granules remain in the developer material without being additionally supplied nor discharged, the quantity of the developer material will not change much because only the quantity of toner particles changes. Therefore, using the toner concentration sensor provided at the position in contact with the developer material, the ratio of the toner particles to the developer material can be accurately controlled.
On the other hand, in the case where carrier granules are additionally supplied to the developer material to gradually replace the carrier granules whose quality has deteriorated, there i a greater change in the quantity of the developer material between directly after the supply of carrier granules and directly after the discharge of the developer material compared with the above case where carrier granules are not additionally supplied. Especially in the case of a portable type compact copying machine, due to an impact or tilt when moving, a large quantity of developer material may be discharged through the discharge opening at one time. Therefore, a constant quantity of the developer material in the developer vessel is difficult to maintain.
Furthermore, because the quantity of developer material in the developer vessel is not always constant, the ratio of toner particles in the developer vessel cannot be accurately controlled by the magnetic permeability sensor provided in the developer vessel.
In the above developer device, an attempt has not been made to accurately detect the ratio of toner particles to the developer material in the developer vessel by the toner concentration sensor. Therefore, in the case where the toner concentration sensor is merely provided so as to be in contact with the developer material as in the case of the conventional model, the ratio of the toner particles to the developer material in the developer vessel may vary greatly in the vicinity of the toner concentration sensor due to a change in the quantity of the developer material. Specifically, when the concentration of the developer material in the vicinity of the toner concentration sensor is high, additional toner particles may not be supplied, thereby presenting the problem that the sufficient quantity of toner particles may not be ensured. On the other hand, when the concentration of the developer material is low, toner particles may be supplied excessively, thereby presenting the problem that the ratio of toner particles to the developer material becomes too high. In the above cases, because a copying operation cannot be carried out with an appropriate ratio of the toner particles, the image quality may deteriorate.
Another type of developer device has been proposed wherein the toner concentration sensor is provided on the bottom wall of the developer vessel. However, the above device also presents the problem that the ratio of the toner particles to the developer material may not be detected accurately. For example, in the case where the toner concentration sensor is provided on the bottom wall under the developer roller, in the position where carrier granules are moved thereto after supplying toner particles onto the photoreceptor for the developing process, the ratio of the toner particles in the developer material tends to be low. Thus, it is difficult to detect the average ratio of the toner particles in the developer material stored in the developer vessel. Moreover, depending on the structure of the developer vessel, in the case where the toner concentration sensor is provided on the bottom wall, the toner concentration sensor may not be always in stable contact with the developer material. Therefore, the above arrangement also fails to overcome the problems raised in conventional models.